The present invention relates to a laser module for optical information processing to record or read information recorded onto an optical recording medium such as an optical disc, a photomagnetic disc, or the like, with laser light, and particularly relates to a laser module for dealing with a plurality of wavelengths for the combination of DVD and CD or the like, an optical head and an optical information recording and reproducing apparatus using it.
Recently, a DVD (Digital Versatile Disc) drive using laser light having a wavelength of 650 nm has come into a wide use rapidly as a dual-wavelength drive which is also applicable to CD using laser light having a wavelength of 780 nm. In an optical head used in this drive, semiconductor lasers, collimating lenses, objective lenses, photodetector, etc. are mounted for the respective CD and DVD in order to keep compatibility between DVD and CD. As a result, the number of parts is doubled, and their optical adjustment is complicated, so that the cost increases.
On the other hand, a blue laser having a wavelength of 410 nm has been lively developed, and expected to be put into practical use in the near future. It is therefore necessary to develop a three-wavelength optical head which can also deal with a blue laser while keeping compatibility with CD and DVD. However, preparing and assembling parts for the three wavelengths causes not only a large increase in the cost but also a serious problem in downsizing of the optical head and in ensurance of reliability.
As a technique for miniaturizing such a three-wavelength optical head and ensuring its reliability, JP-A10-21577 discloses an example of a three-laser module in which three semiconductor laser chips are pasted on a silicon substrate having micro-mirrors and photodetectors, and formed into a module. However, when a deflection/diffraction grating is used in common, a photodetector portion for dealing with the three wavelengths spreads over a wide area. As a result, there occurs a problem that the silicon substrate is made large in size so that the cost is increased. For example, FIG. 5 shows a state of a silicon substrate in which three laser chips each having a chip width of 0.25 mm are arranged. A recess portion on the silicon substrate for arranging the chips therein (hereinafter, referred to as "sink portion") is 1.1 mm wide, and focusing and tracking photodetectors corresponding to the respective lasers are disposed on opposite sides of the sink portion. Finally, the width of the silicon substrate reaches 3.9 mm, which is, for example, approximately twice as large as the width of a silicon substrate currently used in a laser module for CD. JP-A-11-134702 disclose a three-wavelength diode for oscillating three-wavelength laser light as a technology corresponding to three-wavelength optical head, and an example for introducing three-wavelength laser light into a common photodetector by using deflector/diffraction grating. However in this situation, two out of three-wavelength are incident into converging optical system in a slanted manner inviting coma aberration.